Many outboard motors are equipped with a tilt and trim mechanism for tilting the motor to an up position for launching, retrieving, servicing or trailering, and for tilting the motor to a down position for use in propelling a marine vessel such as a boat. During boat operation the tilt and trim mechanism can be used to adjust the trim of the motor for optimum boat performance and handling, or for other reasons such as shallow water. Such mechanisms typically include a DC motor driven hydraulic pump and one or more hydraulic piston-cylinder assemblies connected between a mounting bracket, such as a clamp, that is secured to the transom of the boat and a motor mount that is pivotally connected to the mounting bracket.
During normal operation the lower end of the motor's drive shaft housing will extend to a level lower than the bottom of the boat where it might encounter an underwater obstruction such as a submerged log or rock. To protect the motor from damage, and particularly the propeller at the bottom end of the drive shaft housing, a hydraulic tilt piston-cylinder assembly is provided with a shock absorbing/dampening feature that allows the motor to pop up and clear of the underwater obstruction. This is typically provided by pressure relief valves built into the piston of the assembly. A problem arises if the rapid pressure buildup in the assembly causes the piston to bottom out and transfer large loads to the motor's structural components with resultant damage to such components.
Attempts have been made to provide a hydraulic tilt piston-cylinder assembly that reduces the shock at the end of travel of the piston. One such attempt is disclosed in U.S. Patent Application Publication No. 2005/0090163 wherein a spring-biased oil lock piston defines with the cylinder wall an annular restricted flow path that meters the flow of hydraulic fluid therepast as it is being pushed by primary piston in the cylinder.